Previous research has not examined moment-to-moment mood in relation to specific risky behavioral events among depressed adolescents, who have a markedly elevated risk of HIV and other sexually transmitted infections (STIs). To assess these complex relationships, this New Investigator-led pilot study proposes using innovative momentary sampling methods and draws on studies and theories of mood, cognition, and behavior. Specific aims are (1) to demonstrate the feasibility of recruiting depressed adolescents to participate in a momentary sampling study of mood and HIV/STI risk behavior, (2) to examine the associations of antecedent, concurrent, and subsequent positive and negative mood, and mood variability with sexual thoughts, desires, and behaviors among depressed adolescents, and (3) to describe gender differences in these associations. A gender-balanced sample of 76 depressed sexually active outpatients 15- 22 years of age will be recruited from adolescent and psychiatric clinics. Baseline data will be collected using audio computer-assisted self-interview (A-CASI) and a timeline follow-back (TLFB) calendar to recall coital and substance use events over the previous week. Participants will complete two weeks of electronic diaries using a handheld computer to respond to random signals with reports of their moods, social context, thoughts, urges, and risk events. Participants also will be asked to initiate a report after coital and substance use events. At the end of the two weeks, participants will recall average moods by A-CASI and specific moods and behaviors by TFLB calendar for the same period. Data will be analyzed to examine associations of antecedent moods, average moods, and mood variability with HIV/STI risk, and whether these associations vary by gender, personality factors, and substance use. Enhancing our understanding of the associations of mood and risk behavior among depressed adolescents will be essential to the development ns designed specifically for this at-risk population.